1. Field of the Invention
The field of this invention relates to an improved process of recovering the sulfur from Claus tail gases.
2. Prior Art
The recovery of elemental sulfur by the Claus reaction is well known and various processes using this reaction are in commercial use. The Claus reaction involves obtaining elemental sulfur from a gas that contains sulfur dioxide and hydrogen sulfide according to the reaction: EQU 2 H.sub.2 S + SO.sub.2 .fwdarw. 2H.sub.2 O + 3 S
in the Claus process one strives to obtain as good as possible a stoichiometric ratio of hydrogen sulfide to sulfur dioxide of 2 moles hydrogen sulfide to 1 mole of sulfur dioxide in order to make the yield of sulfur according to the Claus reaction be as large as possible. Usually the reaction is carried out in steps, namely one thermal step, whereby a great portion of the sulfur is formed and is condensed out by cooling of the gas, followed by two or more catalytic steps at elevated temperature, with intermediate cooling of the gas for condensation of the sulfur formed.
The Claus reaction, however, never proceeds quite completely. Even if the ratio of the gases employed therein is substantially stoichiometric, the Claus tail gas still will contain some sulfur dioxide and/or hydrogen sulfide. It is also possible that some carbonyl sulfide and/or carbon disulfide can form in the course of the Claus reaction and appear in the tail gas. The result is that the effluent or tail gas stream from such a reaction can contain substantial quantities of sulfur compounds.
It has been recognized that emitting this tail gas to the atmosphere can present environmental problems because of the sulfur content.
A known method for recovering the sulfur content of such a tail gas stream involves enriching this effluent gas stream with a source of hydrogen to a level which is at least equal to the stoichiometric amount of hydrogen required to convert the contained sulfur dioxide and other sulfur compounds to hydrogen sulfide and catalytically hydrogenating essentially all of the contained sulfur compounds to hydrogen sulfide at a temperature from about 300.degree. to about 800.degree. F., and treating the hydrogenated gas stream to remove hydrogen sulfide.
A variation of this method involving specific effluent gas streams is disclosed in U.S. Pat. No. 3,752,877 to Beavon issued Aug. 14, 1973, incorporated herein by reference.
Such a process can be desirable because the hydrogen sulfide can be conveniently collected employing conventional procedures and recirculated to the Claus reactor.
In such a process, the tail gas and hydrogen must be heated to a temperature sufficient to effect the conversion of sulfur compounds to hydrogen sulfide.
A very advantageous method of supplying this heat entails combusting a hydrocarbon gas and mixing the hot combustion products directly into or with the tail gas and hydrogen, for example, via an in-line burner. Heretofore, however, this method of supplying heat has not provided good process results. Adverse results have included carbon formation on the catalyst, and formation of undesirable sulfur-containing reaction products, i.e., sulfur reaction products other than hydrogen sulfide.
A more desirable process would include in-line heating without the adverse results mentioned above.